1. Field of the Invention
The present invention relates to a method for producing a conductive polymer solid electrolytic capacitor, and, particularly, to a technique for preventing an oxidant from creeping up along an anode wire when an electrolyte is formed by chemically oxidative polymerization and to a method for uniformly forming the film thickness of an electrolytic conductive polymer.
2. Description of the Related Art
There has been an increasing demand for a small-sized and high capacity capacitor which is reduced in impedance in a high frequency range along with a development of light weight and miniaturized electronic equipment. In general, examples of a small-sized and large capacity capacitor are aluminum electrolytic capacitors and tantalum electrolytic capacitors.
The aluminum electrolytic capacitors have a considerable advantage in preparing low cost and large capacity capacitors. However, those capacitors have the drawback of capacitive deterioration due to vaporization of a liquid electrolyte with time because of the use of the liquid electrolyte and the drawback of impaired high frequency characteristics. On the other hand, the tantalum electrolytic capacitors have been improved in the drawback of, for instance, such an impaired capacitive deterioration as is seen in the aluminum electrolytic capacitors by using solid manganese dioxide as the electrolyte.
The solid electrolyte (manganese dioxide) used for the tantalum electrolytic capacitor is formed by making an aqueous manganese nitrate solution impregnated with and adhere to a dielectric oxide film, followed by heat decomposition. This causes the dielectric oxide film, e.g., tantalum oxide film, to be damaged during the heat decomposition and also causes high equivalent series resistance (ESR) at high frequencies.
In order to solve these drawbacks, a proposal has been made on a solid electrolytic capacitor using, as a solid electrolyte, a conductive polymer produced by polymerizing, for example, a complex five-membered ring compound, such as pyrrole, thiophene or furan, which has a higher conductivity than manganese dioxide.
Examples of a method for producing a solid electrolytic layer of the aforementioned conductive polymer are a method of electrolytically polymerizing a monomer and a method of polymerizing a monomer by chemical oxidation. As the electrolytic polymerization method, for instance, a method is well-known in which a mixture solution consisting of a polymerizable monomer and a supporting electrolyte is used and voltage is applied to the mixture solution to polymerize. As the chemically oxidative polymerization method, a method is well-known in which a polymerizable monomer is mixed with an oxidant primarily in a liquid phase. The present invention resides in a method for producing a solid electrolytic capacitor in which method a solid electrolytic layer of a conductive polymer is formed by the latter chemically oxidative polymerization between the two polymerization methods.
Japanese Patent Application Laid-Open (JP-A) No. 3-155110 discloses a method in which a capacitor element which has been finished as far as the formation of a dielectric oxide (e.g., tantalum oxide) film is immersed in an alcoholic solution of an oxidant to allow the oxidant solution to adsorb to the dielectric oxide film and thereafter a monomer solution is applied in the case of forming a solid electrolytic layer of a conductive polymer to produce a solid electrolytic capacitor by using chemically oxidative polymerization.
Also, JP-A No. 7-118371 discloses a technique for polymerizing an electrolyte consisting of aniline and a copolymer compound by a chemical oxidation process in which a tantalum pellet treated by anodic oxidation is immersed in a solution produced by dissolving para-toluene sulfonic acid and an oxidant in a mixture solution of water and ethyl alcohol immediately after it is immersed for a given period of time in a solution produced by dissolving para-toluene sulfonic acid, aniline and pyrrole in a mixture solution of water and ethyl alcohol.
However, when an alcoholic solution of an oxidant is used like in JP-A No. 3-155110, as shown in FIG. 1, an oxidant 22 creeps up along an anode lead 21 led out of a capacitor element 20. Then, as shown in FIG. 2, an electrolyte (cathode side) 23 is formed by polymerization on the anode lead 21, giving rise to the drawback of a development of a short-circuit between the electrolyte 23 and an anode terminal 24 connected to the anode lead 21.
In the technique of JP-A No. 7-118371, in turn, the oxidant creeps up along an anode lead, posing the problem that an electrolyte is formed on the anode lead. There is also the problem that an electrolyte cannot be formed uniformly on the surface of the tantalum pellet treated by anodic oxidation.
JP-A No. 5-166681 discloses a technique for preventing an electrolyte from creeping up along an anode lead by forming a water-repellent masking on a lead portion of the anode lead. JP-A No. 7-201662 discloses a technique for preventing a conductive polymer reaction solution from creeping up along an anode lead by forming a block material using, for instance, a fluororesin or a silicone resin on the lead portion of an anode lead of a capacitor element such that the angle formed by the block material with the surface from which the anode lead is led is designed to be smaller than the contact angle between the conductive polymer-forming reaction solution and the block material.
JP-A No. 5-166681 in which the water-repellent masking is formed on the lead portion of the anode lead has an effect of preventing an electrolyte from creeping up along the anode lead to some extent. This technique, however, has the problem that it can imperfectly prevent an electrolyte from creeping up along the anode lead. While, JP-A No. 7-201662 has an effect of preventing the conductive polymer reaction solution from creeping up along the anode lead by forming a block material into a specific shape and arranging it on the lead portion of the anode lead of the capacitor element. This technique, however, has the problem of an increase in cost for manufacturing the block material.
When 100 wt % water medium is used as the solvent for oxidant, the dispersion of the thickness of the conductive polymer film to be formed is increased, giving rise to the problem of increased dispersion of the ESR characteristics, though the application of the aforementioned repellent agent has an effect of preventing creeping-up.
It is an object of the present invention to provide a method for producing a solid electrolytic capacitor, the method suppressing the creeping-up of an oxidant, improving the electric characteristics between an electrolyte and an anode terminal and decreasing the dispersion of the ESR characteristics of a capacitor.
According to one aspect of the present invention, a method for producing a conductive polymer solid electrolyte capacitor comprises the steps of: forming an anode lead led out of an anode member; forming a dielectric film at a surface of the anode member by anodic oxidation to form a capacitor element; applying a water-repellent agent to a predetermined position of the anode lead; immersing the capacitor element in an oxidant solution of a mixture solvent of alcohol and water; drying the capacitor element; and immersing the capacitor element in an alcoholic solution of a conductive polymeric monomer to polymerize a conductive polymer electrolyte on a surface of the capacitor element by chemical oxidation.
As to, especially, the mixing ratio of alcohol to water in the oxidant solution used in the present invention, preferably the ratio of water to a solvent is defined in a range between 10 and 80 wt %. This can suppress the creeping-up of the oxidant along the anode lead due to the synergetic effect of the oxidant solution and the water-repellent agent applied to the anode lead. Also, the alcoholic solution of the conductive polymeric monomer is brought into contact with the oxidant fixed to the dielectric film to thereby form a conductive polymer film on the surface of the dielectric film by chemically oxidative polymerization.